Deny missing documentation in wasmtime crate (#836)

* Deny missing documentation in `wasmtime` crate

Everything is largely documented now, so let's be sure to keep it that
way!

* Add windows docs

crates/api/src/ref.rs

@@ -1,3 +1,5 @@
+#![allow(missing_docs)]
+
 use std::any::Any;
 use std::cell::{self, RefCell};
 use std::fmt;